From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C9066C35669 for ; Sun, 23 Feb 2020 01:24:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A2AE4206EF for ; Sun, 23 Feb 2020 01:24:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727366AbgBWBYR (ORCPT ); Sat, 22 Feb 2020 20:24:17 -0500 Received: from szxga05-in.huawei.com ([45.249.212.191]:10672 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726934AbgBWBYR (ORCPT ); Sat, 22 Feb 2020 20:24:17 -0500 Received: from DGGEMS414-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id D1DEDE6A910DC861A4E4; Sun, 23 Feb 2020 09:24:12 +0800 (CST) Received: from [127.0.0.1] (10.177.246.209) by DGGEMS414-HUB.china.huawei.com (10.3.19.214) with Microsoft SMTP Server id 14.3.439.0; Sun, 23 Feb 2020 09:24:03 +0800 Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset() To: Matthew Wilcox , Qian Cai CC: , , , , , , , , , "Sean Christopherson" , References: <20200222170222.GJ24185@bombadil.infradead.org> From: "Longpeng (Mike)" Message-ID: Date: Sun, 23 Feb 2020 09:24:01 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20200222170222.GJ24185@bombadil.infradead.org> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.177.246.209] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2020/2/23 1:02, Matthew Wilcox 写道: > On Sat, Feb 22, 2020 at 02:33:10PM +0800, Longpeng (Mike) wrote: >> 在 2020/2/22 13:23, Qian Cai 写道: >>>> On Feb 21, 2020, at 10:34 PM, Longpeng(Mike) wrote: >>>> >>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>> index dd8737a..90daf37 100644 >>>> --- a/mm/hugetlb.c >>>> +++ b/mm/hugetlb.c >>>> @@ -4910,28 +4910,30 @@ pte_t *huge_pte_offset(struct mm_struct *mm, >>>> { >>>> pgd_t *pgd; >>>> p4d_t *p4d; >>>> - pud_t *pud; >>>> - pmd_t *pmd; >>>> + pud_t *pud, pud_entry; >>>> + pmd_t *pmd, pmd_entry; >>>> >>>> pgd = pgd_offset(mm, addr); >>>> - if (!pgd_present(*pgd)) >>>> + if (!pgd_present(READ_ONCE(*pgd))) >>>> return NULL; >>>> p4d = p4d_offset(pgd, addr); >>>> - if (!p4d_present(*p4d)) >>>> + if (!p4d_present(READ_ONCE(*p4d))) >>>> return NULL; >>> >>> What’s the point of READ_ONCE() on those two places? >>> >> As explained in the commit messages, it's for safe(e.g. avoid the compilier >> mischief). You can also find the same usage in the ARM64's huge_pte_offset() in >> arch/arm64/mm/hugetlbpage.c > > I rather agree with Qian; if we need something like READ_ONCE() here, > why don't we always need it as part of pgd_present()? It seems like an > unnecessary burden for every user. > Hi Matthew & Qian, Firstly, this is NOT a 'blindly copy', it's an unwise words. I don't know whether you read the commit message (commit 20a004e7) of ARM64's huge_pte_offset ? If you read, I think worry about the safe is necessary. Secondly, huge_pte_offset in mm/hugetlb.c is for ARCH_WANT_GENERAL_HUGETLB, many architectures use it, can you make sure there is no issue on all the architectures using it with all the version of gcc ? Thirdly, there are several places use READ_ONCE to access the page table in mm/* (e.g. gup_pmd_range), they're also generical for all architectures, and they're much more like unnecessary than here, so why there can use but not here? What's more, you can read this commit 688272809. -- Regards, Longpeng(Mike)